1. Field of the Invention
The present invention relates to an optical transmission system, and more particularly, to a system for optically transmitting a digital modulation signal obtained by frequency-multiplexing.
2. Description of the Prior Art
FIG. 9 is a diagram showing a first example of a conventional optical transmission system. In FIG. 9, the optical transmission system includes an analog-to-digital converting portion 901, a digital modulating circuit 903, an electrical-optical converting portion 905, an optical transmission channel 906, an optical-electrical converting portion 907, a transmission channel 908, a demodulating portion 9100, and a digital-to-analog converting portion 913. The demodulating portion 9100 includes a filter 910 and a digital demodulating circuit 911.
Description is now made of operations of the conventional optical transmission system shown in FIG. 9. The analog-to-digital converting portion 901 samples and quantizes an analog signal, such as an image signal, and converts the sampled and quantized analog signal into a digital signal. The digital modulating circuit 903 converts the digital signal into a digital modulation signal by a predetermined digital modulation system using a carrier having a predetermined frequency f.sub.0, and outputs the digital modulation signal. The digital modulation signal is converted into an optical signal in the electrical-optical converting portion 905, and the optical signal is transmitted by the optical transmission channel 906, and is then reconverted into an electrical signal in the optical-electrical converting portion 907. In the demodulating portion 9100, the digital modulation signal, having the frequency f.sub.0 and which was passed through the filter 910, is converted into a digital signal by the digital demodulating circuit 911. The digital-to-analog converting portion 913 reconverts the digital signal into an analog signal. Examples of the above-mentioned predetermined digital modulation system include a 16 QAM modulation system. In this case, it is generally possible to ensure a transmission capacity of several tens of megabytes per second.
FIG. 10 is a diagram showing a second example of a conventional optical transmission system. In FIG. 10, the optical transmission system includes an analog-to-digital converting portion 901, a dividing portion 1002, a modulating portion 1003, a multiplexing portion 1004, an electrical-optical converting portion 905, an optical transmission channel 906, an optical-electrical converting portion 907, a transmission channel 908, a branching portion 1009, a demodulating portion 1010, a synthesizing portion 1012, and a digital-to-analog converting portion 913. The modulating portion 1003 includes a first digital modulating circuit 10031 and a second digital modulating circuit 10032. The demodulating portion 1010 includes a first filter 10101, a second filter 10102, a first digital demodulating circuit 10111, and a second digital demodulating circuit 10112.
Description is now made of operations of the conventional optical transmission system shown in FIG. 10. This example illustrates a construction in a case where the transmission rate is higher than that in the above-mentioned first conventional system. In this example, two carriers having predetermined frequencies f.sub.1 and f.sub.2, which differ from each other, are used to transmit one signal. Specifically, the dividing portion 1002 divides a digital signal, after analog-to-digital conversion, into two groups of digital information (first digital information J.sub.1 and second digital information J.sub.2) in accordance with a predetermined dividing method. The first digital modulating circuit 10031 and the second digital modulating circuit 10032 in the modulating portion 1003 respectively convert the two groups of digital information, obtained by the division, into digital modulation signals using two independent carriers, having frequencies f.sub.1 and f.sub.2. The multiplexing portion 1004 frequency-multiplexes the digital modulation signals to obtain one signal.
The signal transmitted through the electrical-optical converting portion 905, the optical transmission channel 906, the optical-electrical converting portion 907, and the transmission channel 908 is branched into two signals in the branching portion 1009, after which the signals are respectively inputted to the first filter 10101 and the second filter 10102 in the demodulating portion 1010. The first digital demodulating circuit 10111 demodulates the first digital modulation signal (having the frequency f.sub.1) which was passed through the first filter 10101 to yield the first digital information J.sub.1. Likewise, the second digital demodulating circuit 10112 demodulates the second digital modulation signal (having the frequency f.sub.2) which was passed through the second filter 10102 to yield the second digital information J.sub.2. The synthesizing portion 1012 synthesizes the first and second digital information J.sub.1 and J.sub.2 in accordance with a process which is reverse to the above-mentioned predetermined dividing method used in the dividing portion 1002, to reproduce a digital signal. Examples of the predetermined dividing method include a method of dividing a digital signal into two groups of digital information every other sample. As described in the first conventional example, there is a limit on the capacity of the digital modulation signal which can be transmitted by one carrier. In this example, therefore, a digital signal, which is to be transmitted, is divided into a plurality of (two in FIG. 10) groups of information, and the groups of information are respectively converted into digital modulation signals using independent carriers. The digital modulation signals are frequency-multiplexed and are together optically transmitted together, thus ensuring a higher transmission rate (which is twice that in the first conventional example).
FIG. 11 is a diagram showing a third example of a conventional optical transmission system. In FIG. 11, the optical transmission system includes first to third analog-to-digital converting portions 11011 to 11013, a dividing portion 1102, first to third modulating portions 11031 to 11033, a channel multiplexing portion 1104, an electrical-optical converting portion 905, an optical transmission channel 906, an optical-electrical converting portion 907, a transmission channel 908, a channel branching portion 1109, first to third demodulating portions 11101 to 11103, a synthesizing portion 1112, and first to third digital-to-analog converting portions 11131 to 11133.
Description is now made of operations of the conventional optical transmission system shown in FIG. 11. The above-mentioned first and second conventional systems are for transmission on one channel, while the third conventional system is for transmission on a plurality of channels (three channels in FIG. 11). In this conventional example, signals on three channels C.sub.1, C.sub.2 and C.sub.3 are subjected to digital modulation using carriers having different frequencies f.sub.1, f.sub.21, f.sub.22 and f.sub.3 which are previously assigned. The modulated signals are frequency-multiplexed and are optically transmitted together, as shown in FIG. 12, for example. In this third conventional example, therefore, an analog-to-digital converting portion, a modulating portion, a demodulating portion, and a digital-to-analog converting portion are provided so as to correspond to each of the three transmission signals C.sub.1, C.sub.2 and C.sub.3. The channel multiplexing portion 1104 frequency-multiplexes digital modulation signals outputted from all the modulating portions 11031 to 11033. The channel branching portion 1109 branches a signal, which is obtained by the frequency-multiplexing and which is optically transmitted through the electrical-optical converting portion 905 to the transmission channel 908, and respectively inputs signals obtained by the branching to all the demodulating portions 11101 to 11103. In FIG. 11, only the signal on the second channel C.sub.2 requires a high transmission rate. Therefore, the dividing portion 1102 and the synthesizing portion 1112 are provided on the second channel, and transmission is made using the two carriers (having the frequencies f.sub.21 and f.sub.22). Detailed operations of components are the same as those in the first and second conventional examples and hence, the detailed description thereof is omitted.
A system using an analog SCM (Sub-Carrier Multiplex) transmission technique for optically transmitting a signal obtained by frequency-multiplexing using a plurality of carriers as described above is effective as a system capable of easily transmitting information at a high bit rate and on multi-channels. However, a transmission system (including an electrical-optical converting portion, an optical transmission channel, an optical-electrical converting portion, and a transmission channel) generally has frequency characteristics which make it difficult to obtain transmission qualities which are equal and uniform over the entire transmission band. For example, a light source for direct modulation (a semiconductor laser) used for an electrical-optical converting portion exhibits a larger amount of waveform distortion at higher modulation frequencies. This characteristic is due to the effect of the laser's relaxation oscillation frequency. Therefore, degradation of the waveform of a digital modulation signal having a high carrier frequency is larger than degradation of the waveform of a digital modulation signal having a low carrier frequency, whereby the transmission quality such as BER (a bit error rate) is degraded. Further, when the number of transmission channels is large, second order distortion is large in the vicinity of the center of the transmission band, while third order distortion is large in the upper and lower parts of the transmission band.
In the conventional optical transmission system for transmitting a plurality of digital modulation signals using the analog SCM transmission technique as described above, the degree of degradation of the waveform of a transmission signal or the magnitude of distortion thereof differs depending on the carrier frequency, whereby there arises a difference in transmission quality among channels.
The above-mentioned problems not only arise in when an analog signal is converted into a digital signal and the digital signal is transmitted, but also when digital data is multiplexed and optically transmitted.